Wireless communication systems are ubiquitous for personal and commercial uses.
Demand continues to increase due to an increase in quantity of users and an increase in quantity of data desired (e.g., graphics, video, data, etc.). However there is a limit to the number of signals a communication system can accommodate per the number of orthogonal or quasi-orthogonal codes, for a direct sequence spread spectrum application. This is because the communication system is an interference limited and/or a code limited resource.
The need arises to accommodate the increase in the quantity of users and quantity of data desired by the users with the limited resources of the communication system.
One method of accommodating higher data rates is to use wideband transmissions, e.g., three data streams, that are combined at the receiver to produce the resultant signal. However, given the limitation in numbers of code sequences, a wideband system will simply consume the limited number of code resources faster.
Consequently, a need arises to provide wideband transmission without the limitation of consuming code resources.
In a DSSS communication system, multiple signals with different encoding sequences are transmitted simultaneously. To retrieve the desired data stream from the overall data signal, the specific code sequence used to encode the desired signal is reproduced at a receiver, and via the autocorrelation properties, used to detect the original data stream from the noise of and interference in the overall signal. However, in a system such as this, the multiple signals must be sufficiently weak to appear as noise when compared to the signal detected after correlating with the specific code sequence.
Alternative paths exist between a transmitter and receiver due, e.g., to different reflections from objects such as buildings, mountains, trees, cars, etc. that provide duplicate signals at the receiver with unique time delays. These alternative paths or multipaths can be demodulated at the unique time delays and added up to improve the signal-to-noise ratio (SNR). However, in some cases, many or all of the multipaths provide very weak signals due to interference from other transmitters.
Consequently a need arises to overcome the limitation of signal reception due to interference of other transmitters.
One method used to overcome this limitation is to use multiple antennas on a transmitter alone or to use multiple antennas on a transmitter and receiver. This provides additional multipaths for the signal that might overcome some of the geographical barriers as well as some interference suppression capability to the receiver. However, if different signals transmitted on the different antennas use different codes, then the code resource is used up quickly. Consequently, this model accomplishes little gain in the data rate.
Thus, a need arises to overcome the limitation of using different code sequences to encode data for each of multiple antennas.
Capacity is limited by the number of available codes. In particular, orthogonal or at least quasi-orthogonal code sequences must be used for each unique data stream being communicated. However, the number of orthogonal or quasi-orthogonal code sequences is limited for a given code sequence length.
Thus the capacity of the communication system is limited. Consequently, a need arises for a method to satisfy additional transmission capacity while overcoming the limitation of different encoding sequences required for each data stream.